CRAC Class B (2025) — Questions 581–590
U0855. When participating in VHF amateur radio contests or EME communication experiments, we may need to add an external RF preamplifier for the receiver. Where in the whole transceiver system should this device normally be installed?
参加VHF业余无线电竞赛或进行EME通信实验时，我们可能需 要为接收机添置外置式射频前置放大器。这种设备通常应安装 在整个收发信系统的什么地方？
Exam tips · 考试提示
A receive preamplifier (LNA) must sit as close to the antenna as possible — between the antenna and the receiver — so it boosts the weak signal before feed-line and receiver noise are added (A). Placing it after the PA (B), at the tuner (C), or at audio output (D) would not improve receive sensitivity and could damage it.
U0856. Where in the amateur radio equipment should an external low-pass filter used to enhance rejection of spurious emissions be installed?
用来增强滤除杂散发射的外置低通滤波器应装在业余无线电设 备的什么地方？
Exam tips · 考试提示
A low-pass filter that suppresses spurious/harmonic emissions belongs in the RF transmit path, between the transmitter and the antenna, so unwanted products are removed before they radiate (A). It has nothing to do with the receiver (B), power supply (C), or microphone (D).
U0857. When conducting multi-operator multi-station HF contests, to prevent as much as possible the transmitter's harmonic interference with receivers operating on doubled-frequency bands, we can series-connect an LC low-pass or band-pass filter matched to the operating band between the transmitter and antenna. Regarding such a filter, which statement is correct?
开展多人多机HF竞赛时，为了尽力预防发射机的谐波干扰工作 在倍频频段的接收机，我们可在发射机与天线之间串联一个与 操作波段相匹配的LC低通或带通滤波器。关于这样的滤波器， 以下说法正确的是：
Exam tips · 考试提示
A higher-order LC filter has a steeper roll-off, so it rejects harmonics (e.g., 2nd harmonic into the doubled band) more strongly (A). Lower order (B) is weaker; order does not reduce loss (C is false — higher order usually adds a little loss) and does not by itself raise power handling (D).
U0858. A stub filter, sometimes called a branch-line filter (coax stub), is a band-pass or band-reject filter made from feed line, occasionally seen in amateur radio contest setups. Which of the following descriptions of this filter are correct? (Choose all that apply.)
短截线滤波器有时也叫分支线滤波器（coax stub），是利用 馈线制作的带通或带阻滤波器，时有见于业余无线电比赛设施 中。对于这种滤波器，以下描述正确的有：
Exam tips · 考试提示
A quarter-wave coax stub presents different reactance depending on whether it is shorted or open at the end. A 40 m quarter-wave shorted stub is a high-impedance (parallel- resonant) trap at 40 m/15 m (harmonics) and passes those while shorting 20 m/10 m (A); a 40 m quarter-wave open stub is the complement, passing 20 m/10 m and shorting 40 m/15 m (B). The half-wave versions (C, D) do not match these standard behaviors, so the key marks A and B.
U0859. Some transceivers have an RX ANT connector. This connector is used to:
某些收发信机具有RX ANT接口。这种接口是用来：
Exam tips · 考试提示
The RX ANT jack lets you feed a dedicated receive-only antenna (e.g., a magnetic loop or Beverage) into the receiver, bypassing the transmit antenna for better receive (A). It is not for a second receiver (B), nor for connecting the transmit antenna (C, D).
U0860. Some transceivers have a built-in auxiliary receiver, mainly used for: (Choose all that apply.)
某些收发信机内置了辅助接收机，主要用于：
Exam tips · 考试提示
A built-in sub-receiver is typically used for true split (independent VFO/antenna on a different frequency) (A) and for diversity reception (same frequency, separate antenna, combined to beat fading) (B). I/Q output (C) and IF output (D) are different features (panadapter/SDC interfaces), not the sub-receiver's purpose.
U0861. Diversity reception uses multiple receivers to simultaneously capture the same signal, to obtain more complete information and make the signal easier to discern. But when erecting a separate receive antenna for diversity reception for the auxiliary receiver, note that: (Choose all that apply.)
分集接收用多个接收机同时捕获同一信号，以获取更完整的信 息并使信号更易辩识。但是，为收发信机的辅助接收机架设用 于分集接收的独立接收天线时需注意：
Exam tips · 考试提示
For diversity to work, the two antennas must sample decorrelated signals: space them at least ~1 wavelength apart (A) and use orthogonal polarization (e.g., one horizontal, one vertical) (B) so that when one fades the other is strong. Preamps (C) and preselectors (D) are optional/conditional, not the key requirement; C/D overstate necessity and are not the marked answers.
U0862. When participating in contests or DX contacts, we may need a standalone SDR receiver to assist CW or FT8 decoding software, to obtain more coefficients and improve contact results. If such a receiver is a portable product with a USB interface, we need to prepare:
参加比赛或进行DX联络时，我们或许需要一台独立运行的SDR 接收机来辅助CW或FT8解码软件的运行，以获取更多系数，提 高通联成绩。如果这类接收机是使用USB接口的便携产品，我 们需为之准备：
Exam tips · 考试提示
To run a USB SDR dongle you need a PC plus the SDR control software (e.g., SDR#, HDSDR), the mode decoder (CW/FT8), and a virtual-audio-cable/bridge to route the SDR's audio into the decoder (A). You do not need to write code (B, C) or attach a scope/spectrum analyzer (D); off-the-shelf software suffices. 天线的作用 Antenna Functions
U0431. The function of a transmitting antenna is:
发射天线的作用是：
Explanation · 解析
An antenna (天线) is a transducer: it converts the guided RF current from the transmitter into radiating electromagnetic waves (radio waves). Option A is correct. Gain (B) is a directional focusing effect, not power amplification; an antenna does not generate power. C is a receiver/detector function; D is wrong.
U0432. The function of a receiving antenna is:
接收天线的作用是：
Explanation · 解析
A receiving antenna is the reciprocal of a transmitting one: it intercepts passing radio waves and converts them back into a tiny RF voltage delivered to the receiver input. Option A is correct. Gain (B) focuses, it does not "amplify" free-space waves; audio (C) is produced later by the receiver, not the antenna; D is wrong.